Influence of Liquid Bridge Volume on the Floating Zone Convection

The liquid bridge volume is a critical geometrical parameter in addition to the aspect ratio for onset of oscillation in the floating zone convection. The oscillatory features are generally divided into two characteristic regions: slender liquid bridge region and fat liquid bridge region. The oscillatory modes in two regions are discussed in the present paper.

Transition of oscillatory floating half zone convection from Earth's gravity to microgravity

Oscillatory features of floating half zone convection were experimentally studied by using the drop shaft facility of Japan Microgravity Center which supported microgravity period of 10 s. Coordinated measurements including free surface deformation and oscillation, temperature and flow pattern in both 1-g and micro-g environment were obtained. The oscillatory frequency and amplitude in micro-g condition were lower and larger than the ones in l-g condition, respectively. The results gave, at first time, the oscillatory features such as free surface wave in micro-g, coordinated measurements of more than two physical quantities in the micro-g, and transition of thermocapillary oscillatory convection from I-g to micro-g.

Investigation on a simulation model of floating half zone convection .2. Experiment

A simulation model of floating half zone with non-uniform temperature distribution at the upper rod and uniform temperature distribution at lower rod was discussed by numerical investigation in a previous paper. In the present paper, the experimental investigation of the simulation model is given generally. The results of the present model show that the temperature profile is quite different and the critical applied temperature difference is lower than the one of usual model with same geometrical parameters in most cases. The features of critical Marangoni number depending on the liquid bridge volume are also different from the ones of usual model.

Experimental study of thermocapillary convection on a liquid bridge consisting of fluid with low Prandtl number in a floating half-zone

A device of mercury liquid bridge of floating half-zone is designed to experimentally explore thermocapillary convection and its instability of a low Prandtl number liquid. Noncontacted diagnostic techniques were developed to monitor surface flow and surface deformation. The surface flow and the influence of a growing surface film (or skin) on the flow were observed. It is shown that the film is a key factor in changing the behavior associated with the thermocapillary convection. The experiment indicates that the critical Marangoni number should be much higher than that expected by the numerical simulation. The condition and process of surface film growth are discussed. The surface oscillation of the mercury bridge wrapped with ''dirt-film'' was observed, and the characteristics and the frequency associated with this oscillation are given.

Numerical investigation on a simulation model of floating zone convection

A simulation model with adiabatic condition at the upper rod and constant temperature at the lower rod is studied numerically in this paper. The temperature distribution in a simulation model is closer to the one in the half part of a floating full zone in comparison with the one in a usual floating half zone model with constant temperature at both rods, because the temperature distribution of a floating full zone is symmetric for the middle plane in a microgravity environment. The results of the simulation model show that the temperature profiles and the how patterns are different from those of the usual floating half zone model. Another type of half zone model, with a special non-uniform temperature distribution at the upper rod and constant temperature at the lower rod, has been suggested by recent experiments. The temperature boundary condition of the upper rod has a maximum value in the center and a lower value near the free surface. This modified simulation model is also simulated numerically in the present paper. Copyright (C)1996 Elsevier Science Ltd.

Experiment on the thermocapillary convection of a mercury liquid bridge in a floating half zone

A liquid bridge of a floating half zone consisting of liquid mercury sealed in a glass tube with nitrogen atmosphere was used for the experiment of thermocapillary convection with a low Prandtl number liquid. A non-contacted diagnostic method was developed to monitor the surface flow and the surface oscillation. A growing surface film (or skin) is a crucial source to suppress thermocapillary convection, and is discussed in this paper. For the case of a mercury Liquid bridge, the critical Marangoni number was obtained as 900, and the oscillatory frequency was around 5 Hz.

Numerical simulation of g-jitter effects on half floating zone convection under microgravity environment

The g-jitter effects on the thermocapillary convection in liquid bridge of floating half zone were studied by numerical simulation for unsteady and axi-symmetric model in the cylindrical coordinate system. The g-jitter field was given by a steady microgravity field in addition to an oscillatory low-gravity field, and the effects on the flow field, temperature distribution and free surface deformation were analyzed numerically.

How to determine critical Marangoni number in half floating zone convection

In the present paper, the coordinated measurements of the temperature profile inside the liquid bridge and the boundary variation of Free surface, in addition to other quantities, were obtained in the same time for the half floating zone convection. The results show that the onset of free surface oscillation is earlier than the one of temperature oscillation during the increasing of applied temperature difference, and the critical Marangoni numbers, defined usually by temperature measurement, are larger than the one defined by free surface measurement, and the difference depends on the volume of liquid bridge. These results induce the question, ''How to determine experimentally the critical Marangoni number?'' Copyright (C) 1996 Elsevier Science Ltd.

Fractal features of oscillatory convection in the half-floating zone

Unsteady and two-dimensional numerical simulation is applied to study the transition process from steady convection to turbulence via subharmonic bifurcation in thermocapillary convection of a liquid bridge in the half-floating zone. The results of numerical tests show clearly the fractal structure of period-doubling bifurcations, and frequency-locking at f/4, f/8, f/16 with basic frequency f is observed with increasing temperature difference. The Feigenbaum universal constant is given by the present paper as delta(4) = 4.853, which can be compared with the theoretical value 4.6642016.

Hardware development and ground-based studies for drop shaft experiment on oscillatory convection in half floating-zone

Coordinated measurement of temperature, velocity and free surface oscillation were obtained by using the drop shaft facility for microgravity experiments of half floating zone convection. The ground-based studies gave transition from steady to oscillatory convection for multi-quantities measurement.

G-jitter effects on half floating-zone convection in intermediate-frequency range

The g-jitter influence on thermocapillary convection and critical Marangoni number in a liquid bridge of half-floating rone was discussed in the low frequency range of 0.4 to 1.5 Hz in a previous paper. This paper extended the experiments to the intermediate frequency range of 2 to 18 Hz, which htrs often been recorded as vibration environment of spacecrafts. The experiment was completed on the deck of a vibration machine, which gave a periodical applied acceleration to simulate the effects of g-jitter. The experimental results in the intermediate frequency range are different from that in the low frequency range. The velocity field and the shape of the free surface have periodical fluctuations in response to g-jitter. The amplitude of the periodical varying part of the temperature response decreases obviously with increasing frequency of g-jitter and vanishes almost when the frequency of g-jitter is high enough. The critical Marangoni number is defined to describe the transition from a periodical convection in response to g-jitter to an oscillatory convection due to internal instability, and will increase with increasing g-jitter frequency. According to the spectral analysis, it can be found that the oscillatory part of temperature is a superposition of two harmonic waves if the Marangoni number is larger than a critical value.

Influence of liquid bridge volume on the onset of oscillation in floating-zone convection .1. Experiments

The onset of oscillation in the floating zone convection driven by the gradient of surface tension was experimentally studied, and discussions were concentrated on the influence of liquid bridge volume on the onset of oscillation. Distributions of critical applied temperature difference and frequency depending on the volume of the liquid bridge were obtained, and there was a gap range of liquid volume which separated the curve of marginal stability into two parts for fixed rod diameter and aspect ratio. The results imply that the volume of the liquid bridge is a sensitive critical parameter for the onset of oscillation. The implication on the instability is also discussed in the present paper.

Influence of liquid bridge volume on the onset of oscillation in floating-zone convection .2. Numerical-simulation

The onset of oscillation in the floating zone convection driven by the gradient of surface tension was studied numerically for an unsteady and two-dimensional model, and studies were concentrated on the influence of liquid bridge volume on the onset of oscillation in comparison with the experimental results in the Paper I. The numerical results agree with the experimental ones presented in the previous paper, in which the distributions of critical applied temperature difference depending on the volume of liquid bridge and a gap range of liquid volume in marginal stability curve were obtained.

Influence of buoyancy on floating-zone convection of small bond number

Gravity may influence the velocity and temperature distributions, bouyancy may induce Rayleigh instability and the instability may be excited due to the change of free surface shape associating with gravity in the thermocapillary convection. These effects have been studied in the present paper. The results show that gravity may have an important effect in thermocapillary oscillatory convection even for the cases of small Bond number experiments either on the ground or in space.

Free surface vibration in oscillatory convection of half floating zone

Projecting an orthographical grating mask (20pl/mm) on the surface of a small liquid bridge and receiving the reflected distortion image, one can calculate out reversely the shape of free surface of a liquid bridge. In this way we measured the surface shape of a small floating zone and the two-dimensional deformation of its vibration. The mechanism of thermocapillary oscillatory convection and the three-dimensional variation of the free surface are revealed experimentally. The principle for space experiment has been studied in our laboratory.